What are closures and how do you use them in JavaScript?

Q: What are closures and how do you use them in JavaScript?

Learn the answer to "What are closures and how do you use them in JavaScript?" with detailed explanations, code examples, and best practices on DeployU.

The Scenario

You are a frontend engineer at a social media company. You are writing a new feature that needs to create a private variable that can only be accessed by a specific function.

The Challenge

Explain what closures are in JavaScript and how you would use them to solve this problem. What are the key benefits of using closures?

Wrong Approach

A junior engineer might not be aware of closures. They might try to solve this problem by using a global variable, which would not be a very good solution.

Addresses symptoms, not root cause

Right Approach

A senior engineer would know that closures are the perfect tool for this job. They would be able to explain what closures are and how to use them to create private variables.

Step 1: Understand What Closures Are

A closure is a function that has access to the variables in its outer (enclosing) function’s scope chain. In other words, a closure allows a function to access variables from an outer function, even after the outer function has returned.

Step 2: Create a Private Variable

Here’s how we can use a closure to create a private variable:

function createCounter() {
  let count = 0;

  return function() {
    count++;
    return count;
  };
}

const counter = createCounter();
console.log(counter()); // 1
console.log(counter()); // 2
console.log(counter()); // 3

In this example, the createCounter function returns a new function. This new function has access to the count variable from the createCounter function’s scope, even after the createCounter function has returned.

The Benefits of Using Closures

Benefit	Description
Data Encapsulation	Closures allow you to create private variables that can only be accessed by a specific function.
Stateful Functions	Closures allow you to create stateful functions that can remember their state between calls.
Partial Application	Closures can be used to create partially applied functions.

Systematic, production-ready debugging

Practice Question

What will be logged to the console?

Questions